Incineration of sewage sludge continues to gain more widespread acceptance as a viable treatment strategy to address waste solids generated from wastewater treatment plant operations. A significant number of systems are commercially available and multiple installations exist globally.
Conventional designs for an FBC system such as that shown in FIG. 1, typically utilizes dewatered wastewater solids, produced by a dewatering equipment such as centrifugation or belt filter press located immediately upstream, as a principle fuel source. In some cases, dewatered wastewater solids from multiple remote locations are brought on-site to a central FBC facility and blended to create a homogenous fuel source for the FBC operations.
Additionally, conventional FBC designs utilize intermittent injection of an auxiliary fuel source such as fuel oil or natural gas to minimize sporadic combustion associated with inconsistent sludge feed quality and assist with controlling bed temperature at a level that is above the ignition temperature of the selected auxiliary fuel and feed sludge. When fuel oil is used as the auxiliary fuel source the resulting bed temperature typically ranges from 1200° F. to 1300° F. and when natural gas is utilized the corresponding bed temperature typically ranges from 1300° F. to 1400° F.
At a constant combustion air flow rate, operation of FBC systems is influenced by a number of variable process parameters, several of which are related to the sewage sludge feed including its mass loading and physical/chemical characteristics such as the solids content, volatile content and calorific heating value.
Incineration performance varies in function with the quality of the wastewater solids fuel source. It is generally believed that the quality and consistency of the wastewater solids feed stream is the primary factor in determining the performance of an FBC system.
Specifically, it is well understood that the temperature difference between the freeboard and bed temperatures, referred, to herein as ΔT, varies as a function of the sewage sludge quality. ΔT is known to increase as the solids content of the sewage sludge decreases. ΔT is also an indicator of the degree of over-bed burning, with excessive ΔT values indicating that the level of over-bed burning is too high and, therefore, both limiting plant capacity and increasing emission of pollutants such as CO, organics and NOx compounds.
Fluctuations in sewage sludge quality or loading rate are regular occurrences that result in process “hiccups” or performance excursions and the need for corrective measures such as addition of auxiliary fuel and activation of quench water sprays within the freeboard. Such corrective measures ultimately reduce process capacity and increase operating costs.
Typically, FBC systems are set to maintain a freeboard temperature between 1500 and 1600° F. Quench water sprays, which are generally activated in sequence beginning with initiation of the first spray at 1600° F., are used to prevent exhaust gas temperature excursions and protect downstream equipment such as heat exchangers or waste heat boilers.
To address the regular fluctuation in sewage sludge feed quality, the FBC is typically designed to handle a range of wastewater solids characteristics, frequently resulting in an FBC reactor that is oversized for typical operations and requiring use of auxiliary fuel sources to reach optimal operating temperature, therefore increasing both capital and operating costs.
Efficient operation of an FBC system employs a consistent sewage sludge feed supply to optimize process performance. Therefore, to develop a more efficient and cost effective incineration system, there exists a need to regulate the mass and heat loadings of wastewater solids to the FBC.